1. Field of the Invention
The present invention relates to tire monitoring systems. More particularly, it concerns the use of measurements taken on the tires in view of giving one or several alarms.
2. Description of the Prior Art
The purpose of tire monitoring systems is to alert the driver of any anomaly which could occur to these or to one of these tires, as quickly and as reliably as possible. Among the tire operating parameters to be observed, there is certainly the inflated tire air volume. This can decrease either after a flat, or by natural deflation caused by diffusion through the tire itself, which is never completely impervious to air. Since direct measurement of the volume of air enclosed in the tire is not possible, one accesses it through pressure and temperature measurements. U.S. Pat. No. 4,703,650 describes a coding device which can be used to transmit the tire pressure and temperature from the wheel to the vehicle.
To give the driver reliable and useful information, it is desirable not to be satisfied with displaying the decoded values of the pressure and temperature measurements. Permanent consultation of these indications risks being tiresome and/or their interpretation problematic. These measurements are influenced by various disturbances such as the heat emitted by the brakes or on the contrary the cooling caused by the rim, such as the load transfers which cause slight variations of the volume of the tires, or such as the heat build-up of the tire due to its hysteretic losses.
This is why, in U.S. Pat. No. 4,893,110, a process has been proposed to use measurements based on comparisons between two or more tires which aims at being able to give an alarm in case of failure of a tire without adopting alert thresholds which differ too much from the accuracy attained by the coding/decoding device.
This processing although quite useful, does not allow for taking account of a loss of pressure affecting all tires almost equally, which is most often the case for natural deflation of tires. It is also desirable to do additional specific processing of available measurements for observation of natural deflation.